Linear optical absorption and photoluminescence emission properties of gold nanoparticles prepared by laser ablation technique

Gold nanoparticles of average size varying between 1.1 and 3.3?nm are prepared by 1064?nm Nd:YAG laser ablation of solid gold target kept in ethylene glycol medium. The measured UV-Visible absorption spectra showed the presence of sharp absorption peaks in the UV and in the visible regions due to the interband transition and surface plasmon resonance (SPR) oscillations in Au nanoparticles, respectively. The increase in linewidth of the SPR peaks with the reduction in particle sizes is observed due to intrinsic size effects. The prepared samples exhibit photoluminescence (PL) emissions in the UV-Visible region peaked at ??354?nm due to the recombination of electrons with holes from sp conduction band to d-band of Au. The peak PL intensity in the sample prepared with 60 minutes of laser ablation time is enhanced by a factor of ??2.5 compared to that obtained in the sample prepared with a laser-ablation time duration of 15 minutes.     

Wave-particle duality: The mystery keeps unfolding

The current status of wave particle duality using single photon sources is briefly reviewed On sabbatical leave from S N Bose National Centre for Basic Sciences, Calcutta     

Geometry on the quantum Heisenberg manifold

A class of C∗-algebras called quantum Heisenberg manifolds were introduced by Rieffel in (Comm. Math. Phys. 122 (1989) 531) as strict deformation quantization of Heisenberg manifolds. Using the ergodic action of Heisenberg group we construct a family of spectral triples. It is shown that associated Kasparov modules are homotopic. We also show that they induce cohomologous elements in entire cyclic cohomology. The space of Connes-deRham forms have been explicitly calculated. Then we characterize torsionless/unitary connections and show that there does not exist a connection that is simultaneously torsionless and unitary. Explicit examples of connections are produced with negative scalar curvature. This part illustrates computations involving some of the concepts introduced in Frohlich et al. (Comm. Math. Phys. 203 (1999) 119), for which to the best of our knowledge no infinite-dimensional example is known other that the noncommutative torus.     

Influence of Moon's long-period tide on rainfall in India

Summary The Indian summer monsoon rainfall data for eleven pairs of stations have been subjected to maximum entropy spectral analysis (MESA). The results of the analysis appear to indicate Moon's long-period (18.6y) nodal tidal influence on rainfall. The 10–11 y solar cycle term in rainfall appears to be split up into two components in the spectral results of almost all the stations, as a result of spectral analysis.     

Molecular motion in restricted geometries

Molecular dynamics in restricted geometries is known to exhibit anomalous behaviour. Diffusion, translational or rotational, of molecules is altered significantly on confinement in restricted geometries. Quasielastic neutron scattering (QENS) offers a unique possibility of studying molecular motion in such systems. Both time scales involved in the motion and the geometry of motion can be studied using QENS. Molecular dynamics (MD) simulation not only provides insight into the details of the different types of motion possible but also does not suffer limitations of the experimental set-up. Here we report the effect of confinement on molecular dynamics in various restricted geometries as studied by QENS and MD simulations. An example where the QENS technique provided direct evidence of phase transition associated with change in the dynamical behaviour of the molecules is also discussed.      

Study of width and height of EIT resonance in a Doppler broadened five-level system with varying probe power

We report the experimental observation of electromagnetically induced transparency (EIT) in a Doppler broadened rubidium vapour at room temperature for different probe intensities at a fixed pump intensity in a five-level Λ-type system formed by the D₂ transition of ⁸⁵Rb. For a constant pump intensity, we find that the EIT width and height change with the variation of probe intensity. We observe a nonlinear variation of the height of the EIT peak and a linear variation of the width (FWHM) of the EIT signal with probe intensity. In the Doppler broadened multilevel system, we also observe the velocity selective dips along with the EIT signal. A numerical simulation of the probe response signal based on density matrix representation in a five-level system is carried out to reproduce the experimentally observed spectra.     

Manifestations of drag reduction by polymer additives in decaying, homogeneous, isotropic turbulence

The existence of drag reduction by polymer additives, well established for wall-bounded turbulent flows, is controversial in homogeneous, isotropic turbulence. To settle this controversy, we carry out a high-resolution direct numerical simulation of decaying, homogeneous, isotropic turbulence with polymer additives. Our study reveals clear manifestations of drag-reduction-type phenomena: On the addition of polymers to the turbulent fluid, we obtain a reduction in the energy-dissipation rate, a significant modification of the fluid energy spectrum especially in the deep-dissipation range, a suppression of small-scale intermittency, and a decrease in small-scale vorticity filaments.     

A Continuous Transition Between Quantum and Classical Mechanics. I

In spite of its popularity, it has not been possible to vindicate the conventional wisdom that classical mechanics is a limiting case of quantum mechanics. The purpose of the present paper is to offer an alternative formulation of mechanics which provides a continuous transition between quantum and classical mechanics via environment-induced decoherence.     

Enhancement in soft magnetic and ferromagnetic ordering behaviour through nanocrystallisation in Al substituted CoFeSiBNb alloys

The effect of substituting Al for Si in Co₃₆Fe₃₆Si_4−xAl_xB₂₀Nb₄, (X=0, 0.5, 1.0, 1.5, 2.0 at%) alloys prepared in the form of melt-spun ribbons have been investigated. All the alloys were amorphous in their as-cast state. The onset of crystallization as observed using differential scanning calorimetry (DSC) was found to rise at low Al content up to X=1 at% beyond which there was a decreasing trend. The alloys also exhibited glass transition at ‘T_g’. Microstructural studies of optimally annealed samples indicated finer dispersions of nanoparticles in amorphous matrix which were identified as bcc-(FeCo)Si and bcc-(FeCo)SiAl nanophases by X-ray diffraction technique. Alloy with optimum content of Al around X=1 at% exhibited stability in coercivity at elevated temperatures. Though Al addition is known to lower magnetostriction, such consistency in coercivity may also be attributed towards lowering in the nanoparticle size compared to X=0 alloy. In the nanostructured state, the alloy containing optimum Al content (X=1) exhibited further enhancement in ferromagnetic ordering or the Curie temperature by 100 K compared to alloy without Al. Such addition also attributed to better frequency response of coercivity and low core losses.